is 600cfm really enough?

Jon350 · Oct 25, 2004

Well according to Holley and scientific carb. calculation pages, 600cfm is suppose to be just about right for me. BUT I can't help but feel like the carburetor is choking some performance.
This is my basic setup:
302->306cid
JE forged flat tops, 9.25-9.5:1 cr
E-303 (1.72 scorpoin rockers: .534" lift, 282adv. degrees)
165 AFR's
Performer RPM intake, 600cfm holley 4150, 1"phenolic spacer (4 hole)
hedman shorties (just the 1 1/2" primaries w/ 2 1/2" colletors)
Complete MSD ignition system.......
MY 600cfm holley (vac. secondaries) doesn't feel like it's up to snuff, it's 5 years old and been through hard times. I think it could be slowing my overall motor down, but i've cleaned it out numerous times and can't feel a huge difference aside from idle and startup.
As a side note for the distributor. my mechanic said it would be better for me to leave the (vacuum advance hose off of the distributor/carburetor because of detonation possibilities), so now i just have it plugging up vacuum holes on the carb. .....But it seems to that that could be leaving out too much advance (around 10 degrees)
All in all i'm happy with this daily driver setup but feel like i could be squeezing a little more out of it....I think i need to just tune it on a dyno. Any ideas would help. Thank You.

Old Blue · Oct 25, 2004

for a 302 600 cfm is more than enough. 570 is actually plenty for that block. i dont think you would need to be concerned with more than 600cfm unless you are pushing well beyond 400hp at the fly, even at a little over 400hp you would be fine with a 600. Based on the engine mods you listed i would guess (please dont flame me if i am way off) you are around 300hp at the flywheel at the moment, so 600cfm is fine indeed.

as far as squeezing out more hp, i think all you are lacking to take advantage of the mods you already have is a good cam. that i think will compliment what you have done so far nicely, and to your liking. give compcams an email with the details on your carb, intake, heads, bore/stroke. exaust, trans, rear gear ratio, and they will remommend a cam perfect for your needs.

hope this helps

tygr1 · Oct 25, 2004

NO TIMING ADVANCE?!? Have u checked/tuned the centrifugal advance?

You need some spark advance to get that motor to RUN!

My 68 289 dist. has had the advance aprings replaced with the lightest ones I could get. I time my car with a vacuum gauge and a test drive...it's the ONLY way to get the timing OPTIMIZED for your car!

Have any ??s...ask away. I am NOT the expert here, but I have a little experience.

DOH!!! Didn't see that u had full MSD.

mdjay · Oct 25, 2004

More cfm will give you more top end and less bottom. So if you have a roller motor that you can wind up, move the cfm up. If it revs close to stock, keep the 600!

6Stang7 · Oct 25, 2004

Let's do that math.
cfm = rpm x displacement / 3456 x 0.85
So, cfm = [5500(my guess from what you have) x 306]/3456 x 0.85
We get cfm = 1,836,000/2937.6 = 572.91666666666666 cfm

thehueypilot · Oct 25, 2004

I am running a stroked 351W and at the track I run a 850 w/vac sec, but around town I run a 750 w/vac sec. Your CFM is limited to the smallest obstruction which in your case looks like the heads......but AFR's are great flowing heads. 600 or 650 sounds about right. Your 1/8 miles times will tell you a lot about what it is lacking.

65fastback2+2 · Oct 25, 2004

6Stang7 said:
Let's do that math.
cfm = rpm x displacement / 3456 x 0.85
So, cfm = [5500(my guess from what you have) x 306]/3456 x 0.85
We get cfm = 1,836,000/2937.6 = 572.91666666666666 cfm

So, lemme try mine, I hope I do this right

CFM = [6500(my guess for a roller motor) x 310 (5.0 0.060 over)]/3456 x 0.85 = 495cfm??? How come its less than the regular 302???

6Stang7 · Oct 25, 2004

65fastback2+2 said:
So, lemme try mine, I hope I do this right

CFM = [6500(my guess for a roller motor) x 310 (5.0 0.060 over)]/3456 x 0.85 = 495cfm??? How come its less than the regular 302???

My guess is that your volumetric efficiency (that 0.85) is wrong. If you want to calculate out your VE, then you need to use this formula.
Theoretical cfm = rpm x displacement / 3456 then with that answer, use it in: acutal cfm / theoretical cfm x 100 = VE

mdjay · Oct 25, 2004

Damn guys. I think I need to go back and take some more math

ratio411 · Oct 25, 2004

The formula is for minimum carb needed...
NOT for maximum.

Also, the VE of 85% is very low for a performance build.
A carbed engine built to a good combo can reach 115%...

The whole over-carb and dual plane issues are way overblown.
I put different carbs on my 302, and swapped them between a mild single plane and an RPM intake.
I got better throttle and top end, with no noticable bottom end loss with the large carb and single plane.

I guess the point is that you have to build to your combo, not what the geeks in Hot Rod preach to you.
Dave

6Stang7 · Oct 25, 2004

ratio411 said:
I guess the point is that you have to build to your combo, not what the geeks in Hot Rod preach to you.
Dave

I got those formulas off Edelbrocks website, and you can't argue against dyno results (well, maybe) and the laws of physics. I know it might seem odd or stupid to use math in a situation like this, but you'd be supprised how much math relates to the world and what you can solve with it.

Max Power · Oct 25, 2004

I have seen these formulas for years, and while it covers the amount of flow required to fill cylinders and avacuate them at maximum rpm, in my mind it doesn't address flow rate. And flow rate is what makes a motor special, isn't it?

By real world standards, I have seen 650 and 700 holleys run much stronger on similar 302 set-ups than 600s.

Given the math, a 600 carb from 2000-4000 rp, should feel no stronger than a 2v at 350 cfm. But it does. At 4000 rpm, mathematically, you shouldn't need more than 350cfm, but your a$$ tells you different.

The dyno and the quarter mile never lie. If you have some friends with other size carbs, give em a try.

Jon350 · Oct 25, 2004

Well, even on Desktop dyno if i bring the cfm to 500cfm, put stock exhaust manifolds on and lower the lift values back down to .498, the estimated values still come to 314hp and 354tq.....so 300hp hurts my feelings . But that's ok....and even though the vacuum advance isn't hooked up i still have the mechanical advance. And I have the upgraded AFR valve springs and i can tell you from auto-X experience, it likes to rev clean up to 6200rpm.....like i said i think i need to tune it on a dyno....the two places i can easily grow are the headers and carburetor.....hell, i know the hedman elites will give more power(just lack ground clearance.).......oh well, I continue onward in the search for more power.

D.Hearne · Oct 25, 2004

Jon350 said:
Well according to Holley and scientific carb. calculation pages, 600cfm is suppose to be just about right for me. BUT I can't help but feel like the carburetor is choking some performance.
This is my basic setup:
302->306cid
JE forged flat tops, 9.25-9.5:1 cr
E-303 (1.72 scorpoin rockers: .534" lift, 282adv. degrees)
165 AFR's
Performer RPM intake, 600cfm holley 4150, 1"phenolic spacer (4 hole)
hedman shorties (just the 1 1/2" primaries w/ 2 1/2" colletors)
Complete MSD ignition system.......
MY 600cfm holley (vac. secondaries) doesn't feel like it's up to snuff, it's 5 years old and been through hard times. I think it could be slowing my overall motor down, but i've cleaned it out numerous times and can't feel a huge difference aside from idle and startup.
As a side note for the distributor. my mechanic said it would be better for me to leave the (vacuum advance hose off of the distributor/carburetor because of detonation possibilities), so now i just have it plugging up vacuum holes on the carb. .....But it seems to that that could be leaving out too much advance (around 10 degrees)
All in all i'm happy with this daily driver setup but feel like i could be squeezing a little more out of it....I think i need to just tune it on a dyno. Any ideas would help. Thank You.

First things first------- find you another mechanic, that guy's a dumb ass (or a Chevy lover ) With what you have in your motor, it should be happy as s*%$# running on 87 octane gas, even with 36-38 degrees total advance. Second, change the secondary spring in your 600 before changing carbs. If it's still got the spring it came with from Holley, it's got a heavy "black" spring, which won't open the secondaries till 4-5000 rpms. Put in a white or purple spring ( the big chain auto parts stores carry the Holley spring assortment for the secondary's) Hook the vacuum advance back up to the ported nipple on the carb ( right side of the main metering block) WIth you're combo, you should be putting out an honest 350 hp. That 600 is plenty for the street, I wouldn't go more than a 700 if you change carbs later.

gp001 · Oct 25, 2004

From the book Auto Math Handbook by John Lawlor, they quote Mike Urich (former engineering vice president of Holley and author of Holley Carbureator and Manifolds) as the following

For a street engine Urich recommend assuming a volumetric efficiency of 85 percent

For a racing engine Urich points out the ve can increase to 100% or more.

Carbureators are not made in an infinite variety of sizes, so you're not likely to find one that corresponds exactly with your calculated carbureator cfm. In most cases, you'd probably be wiser to choose a carbureator the first available size down from your calculated figure rather than the first size up. For choosing a carbureator is one area where bigger is not always better

Maybe some of you guys know more than he does.

Street Carb = [(rpm x displacement) / 3456] * 0.85

Racing Carb = [(rpm x displacement) / 3456] * 1.0

pabear89 · Oct 25, 2004

Catstang has a 65 289 block .30 overbore.
66 cast heads,
rv grind cam,
Cheap China made Airgap Intake,
Flowtech long tube ceramic coated headers,
600 Holley stock spring vac sec
stock point system.

Pulls Good off the Bottom end even with the
Punky 2.79 rear gear.
Top end is lacking in Ign, But runs good for a Driver.

Next step it to respring carb secd, elec the ign system and replace the gear with a 3.50 posi.

D.Hearne · Oct 25, 2004

gp001 said:
From the book Auto Math Handbook by John Lawlor, they quote Mike Urich (former engineering vice president of Holley and author of Holley Carbureator and Manifolds) as the following

Maybe some of you guys know more than he does.

Street Carb = [(rpm x displacement) / 3456] * 0.85

Racing Carb = [(rpm x displacement) / 3456] * 1.0

To assume that formula is correct, you also have to assume that ALL engines that displace that cubic inch displacement, also pull the same amount of vacuum, which in the real world, that ain't gonna happen. There are too many variables in real life to assume that formula is correct 100% of the time. You can take two engines that have the same cubic inch displacement, but have different bore & stoke sizes ( with all other components being the same) and and they will not pull the same vaccum at the same rpm. I've run a Holley 600 on a milder 306 than jon350 has, and it performed flawlessly with the 600. And in case your thinkin the secondaries weren't fully open, you'd be wrong there too, cause I tune all my vac secondary Holleys to open the secondary's almost immediately after you nail the gas. A carb rated at 600 cfm doesn't necessarly flow 600 cfm with any motor it's bolted on. it may flow more , or it may flow less depending on the motor. According to that formula, my 331 stroker motor needs no more than a 575 cfm carb. Well tell that to it's face when you nail the gas and open up ALL THREE 250 cfm 2 bbl carbs and it takes off like a scalded dog.

TOM B · Oct 26, 2004

NO ITS NOT ENOUGH. Go with a 750 and it will kick you back in your seat.
I ran many stangs on the track, started with my first 65 stang 289 4 speed when I was 16, the engine was stock with a 600 holley like yours, I ran 15.80 in the quarter, I put on a 750 holley and ran a 15:20 and thats a big jump for a stock engine. Later I modified it with headers ported 289 heads a big cam with 12.5 to 1 pop ups and a tunnel ram with 2 holley 600 carbs. I was a thrill to say the least, it ran 1330 and yes there were alot of suspension mods to get it there. back then, (70,s) there wasnt alot of parts available for racing for a ford so high strenth aftermarket rods and better bottom end parts where out. The engine finally exploded but what a ride.
So LOOSE the carb math and go with a 750.

one2gamble · Oct 26, 2004

a 750 seems a bit much but I wouldnt be shy of using a 600-650. You could go with a road demon 625 to strike a medium

rbohm · Oct 26, 2004

when using a formula to determine carb sizing, you need to use the rpm at which you develope maximum torque, as that is where your volumetric efficiency peaks. for most healthy street engines that is around 4500rpm. a simpler formula i use, and it does very well by me, is to double the engine displacement and multply by .85 for the street, and .95 for a race motor, and that will get you very close to what you need. as far as your problem goes, you just need to tune your combination better. start by setting your initial timing at 10 degrees, assuming the 13 degree slot on your dist advance mechanism that should set you right at 36 degrees total timing(without the vacuum advance). once you have that set, then hook up the vacuum advance. next you need to tune the holley to your engine, lighter secondary springs, a jet change, go down about 4 sizes and that should put you close, and perhaps even a 1" thick 4 hole spacer.

is 600cfm really enough?

Jon350

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tygr1

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mdjay

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